Target aiming system

ABSTRACT

Assessing the accuracy with which a round fired from a gun hits an intended target is achieved and alignment of the gun is corrected by monitoring the target with an image sensor which is associated with a computer enabling a frame by frame analysis and comparison of the fired round&#39;s trajectory with computer-generated alternative trajectories.

FIELD OF THE INVENTION

The present invention relates to target aiming systems.

BACKGROUND OF THE INVENTION

In a battle situation it is necessary for a gun crew to be able toassess the accuracy with which rounds fired by their gun are hittingintended targets. Conventionally, this assessment has been carried outvisually with the aid of binoculars or a telescope. However, visualassessment of this type is of limited use because of the momentarynature of the event being observed and because the resulting cloud ofsmoke and dust which is raised by the resultant explosion can easilyobscure the point of impact. It is also often the case that when anincoming round has landed close to a target such as a tank, the tankcrew will rapidly fire off smoke bombs to obscure them from theattacking gun, again obscuring the view of the observer. Furthermore,the observer's line-of-sight can be interrupted by smoke and dust thrownup by his own gun and by vibration produced on firing the gun.

It is known to use an image sensor (typically a thermal imager) mountedon a gun and directed at the target to record continuously while the gunis being fired. The video sequence recorded can be viewed subsequentlyin an attempt to assess the accuracy of fired round. The gun operatorcan then attempt to correct any gun targeting errors by realigning thegun barrel. However, the transient nature of the firing and impactevents, as well as the relatively small size of a fired round, makes itextremely difficult for the operator to view the trajectory of the roundand the point of impact. The subjective nature of this process leavesopen the possibility of significant human errors being introduced in therealignment stage.

A further disadvantage if this system is that it generates a largeamount of recorded data which must generally be stored on video tape, anunreliable storage medium under battlefield conditions. Whilst solidstate memory may be used, this is expensive where it is required tostore a long video sequence or a large number of sequences to be storedfor later historical analysis. Furthermore, in order to identify thatportion of the video sequence which shows the round passing or hittingthe target, perhaps only one or two frames of the video sequence, thegun crew must review a relatively large number of frames. In a battlesituation, the time wasted studying the sequence can be critical.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome or at leastmitigate the disadvantages of known target aiming systems.

According to a first aspect of the present invention there is provided amethod of correcting the alignment of a gun following the firing of around at a target by the gun, the method comprising the steps of:

aiming the gun at the target and predicting an expected trajectory for around to be fired

firing the gun and monitoring the target and its surrounding area withan image sensor;

predicting a plurality of alternative round trajectories which encompasspossible variations from said expected trajectory;

analysing image data generated by the image sensor to determine which ofsaid trajectories the fired round followed and, if it is determined thatthe fired round followed one of said alternative trajectories,determining a gun alignment correction factor (for use with a subsequentround) from a comparison of the followed trajectory and said expectedtrajectory.

In a preferred embodiment of the present invention, the image sensorprovides a sequence of image frames which together form a video recordof the travel of the fired round and said step of analysing the imagedata comprises normalising the frames to subtract stationery backgroundtherefrom and then for each said trajectory:

mapping the trajectory onto the two-dimensional plane of the imageframes;

for each frame predicting the displacement of a round following thetrajectory, relative to a fixed reference point;

translating the frames of the sequence relative to said fixed referencepoint by the respective predicted displacements;

summing the translated frames to generate a single cumulative frame;

identifying features present in the cumulative frame which exceed athreshold level and which have a form chosen to be indicative of a firedround.

Typically, for the cumulative frame corresponding to the actual roundtrajectory, the fired round will appear as a bright spot, having agaussian intensity distribution.

If for one of the trajectories a feature is identified in the cumulativeimage which exceeds said predetermined threshold then that trajectory isidentified as the trajectory followed by the round. If features are soidentified for a number of different trajectories, then the featurehaving the strongest intensity is selected and the associated trajectoryidentified.

Said video record may contain any appropriate number of image frames andmay encompass a part or all of the travel of the fired round from gun totarget.

The preferred embodiment described above may be modified so that,instead of considering each frame in its entirety, only a portion orpatch of each frame predicted to contain the round, is considered. Thispatch will be of the same extent for each frame and it is only necessaryto translate and sum the identified patches, considerably reducing thecomplexity of the image processing operation.

It will be appreciated that the field of view of the image sensor shouldbe arranged such that it encompasses all or at least a part of each ofthe possible trajectories of a fired round.

According to a second aspect of the present invention there is provideda method of determining the site of impact of a round fired by a gun ata target, the method comprising:

monitoring and recording the target and its surrounding area with animage sensor;

defining a threshold level of change in the output of the image sensoras being indicative of an impact of a round;

following the firing of a round, detecting a change in the output of theimage sensor in excess of said defined threshold and identifying theregion of change; and

determining the centroid of said region of change and identifying thiscentroid as the site of impact of the fired round.

The detected change in the output of the image sensor may be determinedrelative to the preceding image frame in a sequence of image frames.Alternatively, the change may be determined relative to an imagerecorded prior to firing of the round.

According to a third aspect of the present invention there is provided atarget hit assessment method for enabling a gun crew to determine theaccuracy of a round fired by a gun, the method comprising:

estimating prior to firing the time-to-impact of the round, withreference to the time of firing of the gun, from the properties of theround and the gun and the prevailing atmospheric conditions; and

following firing of the gun, commencing recording of a video sequence ofthe target shortly before the estimated time-to-impact of the round andsubsequently stopping recording shortly after the estimatedtime-to-impact; and

playing back the recorded sequence in slow motion on a video display toallow the accuracy of the firing to be quantified.

The method of the above third aspect provides a method which presentsonly minimal data storage requirements which can be satisfied forexample by a compact solid state memory and which, because the recordedvideo sequence represents only a relatively short time window around theestimated time-to-impact, allows the gun crew very quickly to quantifythe accuracy of the round fired.

The length of the video sequence recorded is determined in part by theaccuracy with which the time-to-impact of the round can be estimated.Typically however, the video sequence will comprise less than 50 framesand, more preferably, less than 10 frames. Given the relatively shortlength of the sequence, the sequence can be played back, slowed down bya factor or 20 or more.

It will be appreciated that elements of the above third aspect of thepresent invention may be incorporated into the method of the first andsecond aspects. In particular, from a knowledge of the time of firing ofthe gun, and using the estimated time-to-impact, an estimate of therelative time at which the round will enter the image sensor's field ofview may be made. Searching of the field of view of the image sensor forthe fired round may be commenced only shortly before the estimated entrytime and may be stopped shortly thereafter. Thus, the risk of rogueimages triggering the tracking procedure may be reduced.

According to a fourth aspect of the present invention there is provideda target hit assessment system for enabling a gun crew to determine theaccuracy of a round fired by a gun, the system comprising:

an image sensor having a field-of-view capable of including an intendedtarget;

computer means for estimating the time-to-impact of a round to be firedby the gun with reference to the time of firing of the gun;

video recording means coupled to the image sensor and arranged to recorda video sequence from the image sensor commencing shortly before theestimated time-to-impact of a fired round and stopping shortly after theestimated time-to-impact; and

video display means coupled to the video recording means for receivingtherefrom said recorded video sequence for playback in slow motion.

Preferably, said image sensor is a thermal image sensor which is capableof detecting the hot rear end of a shell or other munition.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and in order to showhow the same may be carried into effect, reference will now be made, byway of example, to the accompanying drawings in which:

FIG. 1 shows schematically a tank incorporating a target aiming system;

FIG. 2 shows in block diagram form the target aiming system of FIG. 1;

FIG. 3 illustrates the timing sequence of the hit assessment system ofFIGS. 1 and 2;

FIG. 4 illustrates predicted and actual trajectories for a round firedfrom a tank at a target;

FIG. 5A illustrates the predicted and actual trajectories of FIG. 4 asviewed from an image sensor mounted on the turret of the tank;

FIG. 5B shows an enlarged detail of FIG. 5A;

FIG. 6 shows a flow diagram of a trajectory identification process;

FIG. 7 shows a flow diagram of an impact detection process; and

FIG. 8 illustrates schematically the organisation of a fire controlsystem for a tank.

DETAILED DESCRIPTION OF THE DRAWINGS

There is shown in FIG. 1 a tank having a thermal imaging sensor 1,operating in the 8-12 micron window (i.e. a portion of the infra-redregion), mounted on the tank's turret near the breach end of the gunbarrel. The image sensor 1 moves with the turret and it is aligned withthe gun barrel so that the sensor's field-of-view includes a target atwhich the gun is aimed. Both the tank gunner 2 and the tank commander 3are seated behind respective video displays 4,5 which, in normal use,display the video images generated by the image sensor. The video fieldrefresh rate, i.e. the rate at which consecutive frames are captured, isnormally 50 per second which allows the tank gunner to initially aim thegun at a target, e.g. using an on-screen cursor or the like.

When the gun is fired, the tank gunner and commander may be able todetermine whether or not a target has been hit by looking at thereal-time displays for a secondary explosion. However, if the target ishit and no such secondary explosion occurs, or the round fired by thegun misses its target, it is unlikely that they will be able todetermine from the real-time display exactly where the round impacted,or by how much it missed the target, particularly as a large plume ofsmoke and dust is likely to be thrown up by the explosion and because ofthe vibration and smoke caused by the action of firing the gun.

In order to enable the accuracy of hit assessment to be increased, theimage sensor 1 is connected to a video processing unit 6 mounted in therear of the tank's turret. The video processing unit 6 is shown in moredetail in FIG. 2 and comprises a video switch 7 which interfaces theimage sensor 1 to the video displays 4,5 and to a field store 8. Thefield store comprises a solid state memory (not shown) which has acapacity of 10 Mbytes, large enough to store 20 frames.

The video switch 7 is controlled by a fire control computer 9, theprimary function of which is to determine the orientation which the gunbarrel should be positioned in, in order to hit a target identified bythe tank's gunner. The identification may be carried out, for example,using a laser targeting system. From the target identification data, andusing data stored regarding the expected velocity and dynamics of theround, the prevailing atmospheric conditions detected by externalsensors, barrel bend etc., the fire control computer 9 is also arrangedto calculate the time-to-impact (t.to.i) of the shell with reference tothe time of firing of a shell.

In normal operation, the video switch 7 is arranged to couple the outputfrom the image sensor 1 to the video displays 4,5 to provide acontinuous display of the target area on these displays. The output fromthe image sensor is not normally provided to the field store 8. From thecalculated time-to-impact data, the fire control computer 9 is able toidentify a relatively short time window during which a fired shell islikely to impact on the target and during which images of the targetneed to be captured. The accuracy with which the impact estimate can bemade is relatively high, normally being to within a few milliseconds,such that the time window need only be in order of 50 to 100milliseconds to ensure that the event is captured. Thus, a short time(e.g. 5 milliseconds) before the estimated impact, the fire controlcomputer 9 sends a signal to the video switch 7 which causes the outputfrom the image sensor 1 to be transmitted to the field store 8 as wellas to the video displays 4,5. The frames captured during the window arestored in the solid state memory of the field store 8. At the end of thetime window, the fire control computer 9 sends a further signal to thevideo switch 7 causing the transmission of the output from the imagesensor 1 to the field store 8 to cease. The timing of this sequence ofevents is illustrated in FIG. 3.

Following firing of the gun, if the tank gunner or the tank commanderwish to assess the accuracy of the firing, they can operate the firecontrol computer 9 to cause the video switch 7 to couple the videosequence stored in the field store 8 to the displays 4,5. The firecontrol computer enables the stored sequence to be played back at anyappropriate rate, e.g. frame by frame or slowed down by a factor of, forexample, 20. With the image sensor 1 having a video field refresh rateof 50 frames per second, and a projectile residual velocity normallybetween the limits of 500 to 1500 meters per second, a round will travelbetween 10 and 30 meters between consecutive frames which is slow enoughto ensure that the tank crew can track the final moments of the flightof the round from the slowed round during playback, particularly whenthe image sensor 1 is an infra-red sensor such that the hot rear end ofthe round will be clearly visible in flight. In particular, the crew canapproximately identify that frame which shows the round in or nearest tothe vertical plane in which the target lies and determine therefrom thepolar distance of the target from the tank. Alternatively, if the roundlands short of its target, the crew can identify the actual point ofimpact of the round and quantify the offset from the target. In eithercase, the information gained can be used to realign the gun barrelbefore a further round is fired at the target.

The hit assessment system described above enables what is essentially amanual gun realignment process to be carried out. There will now bedescribed with reference to FIGS. 4, 5A and 5B an automatic gunrealignment system which makes use of the thermal imaging sensor 1provided on or near the tank turret and which provides automatictracking of a fired round across the field of view of the sensor.

As described above, it is possible for the computer 9 to estimate thetime-to-impact of a fired round using target identification data, datarelating to the expected velocity and dynamics of the round, theprevailing atmospheric conditions, etc. Using these same parameters, itis possible for the computer 9 to predict a trajectory for the round,between the muzzle or exit end of the gun barrel 10 and the target 11which will result in the target being hit. This trajectory is indicatedby the letter A in FIG. 4 which illustrates a possible battlefieldsituation. In practice, certain unpredictable factors may cause theround to deviate from this predicted trajectory A onto some othertrajectory, e.g. as indicated by the letter B in FIG. 4, which resultsin the round missing its target. Trajectory B can be determined from thedata gathered by the image sensor 1.

FIG. 5A illustrates schematically the field of view 12 of the thermalimage sensor 1 mounted on the tank turret. The trajectories A, B shownin FIG. 4 can be mapped onto the 2-dimensional plane of this field ofview as illustrated. From a knowledge of the deviation of the firedround from the predicted trajectory, it is possible for the computer 9to evaluate the extent to which the gun barrel must be realigned inorder to hit the target. For example, of the round falls to the right orleft of the expected trajectory, the azimuthal angle of the gun barrelis corrected and, if the round falls in front of or behind the target,the elevational angle of the barrel is corrected.

Determining the actual trajectory B of a fired round however is not asimple procedure as a relatively large sequence of image frames,generated by the image sensor must be searched for a relatively smallobject moving at high speed. Moreover, other distracting events may beoccurring in the field of view and a part of that field may be obscuredby smoke and/or dust. Rather than conduct an exhaustive search ofsuccessive image sensor frames for a round entering the field of view,therefore, a search is only conducted along the predicted or primarytrajectory A and along a plurality of secondary trajectories C adjacentto the primary trajectory A as illustrated in FIG. 53. The secondarytrajectories C deviate from the primary trajectory A up to a maximumextent which represents a predicted maximum possible deviation of theround from the primary trajectory A.

FIG. 6 is a flow diagram illustrating a process for identifying theactual trajectory B from a number of predicted trajectories A and C. Asequence of image frames depicting the travel of a fired round towardsthe target are recorded and stored in an image sequence store. Thestored image frames are normalised by, for example, subtracting thefirst image frame from each of the subsequently obtained image frames.The resulting normalised image frames contain only data which isindicative of changes occurring relative to the first image frame. Ifnecessary, in order to ensure that the background remains stationary,the image frames may be compensated for gun motion and vibration.

The predicted trajectories are stored in a predicted tracks store. For afirst of the predicted trajectories or tracks, the three dimensionaltrajectory is mapped onto the two dimensional field of view of the imagesensor. This enables the position of a round following the predictedtrajectory to be identified in each of the recorded and stored imageframes.

The process which is used to predict a round's position in each frame ofthe image sequence for a given trajectory employs standard ballistic andprojection geometry calculations. Firstly, standard calculations usinground-ballistics, platform position and attitude, platform motion,environmental conditions, time-of-shot, barrel bend, and image frametiming, are used to determine the position of the round in globalcoordinates. Secondly, standard projection theory calculations are usedto transform predicted round positions in the three dimensional globalcoordinate system to the two dimensional coordinate system of the imagesensor field of view. Thus it is possible to predict the position of theround in each frame of the image sequence.

Only those frames which are predicted as containing a fired round areconsidered. For each frame, the displacement of the round relative to afixed reference point, for example the position of the round in thefirst frame containing the round, is determined. The frames are thentranslated or shifted by an amount corresponding to this displacement.It will be appreciated that if the fired round is actually following thepredicted trajectory then the round in each frame will be translatedback to the fixed reference point. The shifted frames are then summed.Again, it will be appreciated that if the fired round is following thepredicted trajectory then the summed cumulative image frame will containa high intensity ‘integrated’ round signal or feature at the fixedreference point. Similarly, it will be appreciated that if the actualtrajectory does not coincide with the predicted trajectory then theimages of the round in the image frames will not be translated to thesame point and will therefore not sum cumulatively to produce asatisfactory integrated round signal. Rather, the result will be a lowintensity, smudged, sub-image or area surrounding the fixed referencepoint.

The region surrounding the fixed reference point is examined to identifywhether or not a satisfactory round signal is present at that point,which has an intensity exceeding a predetermined threshold intensity.The shape of the signal may also be examined and compared with areference signal which has the expected shape of a round in flight.

The process is then repeated for the second predicted trajectory. If asignal is identified in the resulting cumulative image at the fixedreference point and which exceeds the predetermined threshold (and whichhas the chosen form), then it is compared against the signal identifiedfor the first trajectory (if indeed such a signal was identified). Ifthe subsequently obtained signal is a better match for a shell in flightthan the previously determined signal then the second trajectory isselected as the present best trajectory. Otherwise, the first trajectoryis kept as the best trajectory. This process is repeated in turn for allof the remaining trajectories to determine which of the predictedtrajectories best matches the actual trajectory.

Having determined the actual trajectory which the fired round hasfollowed, a gun alignment correction factor can be determined bycomparing the actual trajectory against the primary predicted trajectoryA. Typically, the deviation of the round from the primary trajectory isdetermined for each image frame of the recorded sequence. A newtrajectory is then calculated which, when the calculated deviations aretaken into account, will result in the primary trajectory A beingachieved when a further round is fired.

Valuable information concerning firing accuracy may be gained bydetermining the precise impact site of a fired round. Providing that theprocess described above is able to track a fired round to impact, theimpact site will be that region where the round is observed to stoptravelling. However, a preferred way of identifying the impact site isto monitor the sensed image, and in particular the region of that imagecontaining the target, for a change indicative of an explosion. Thenumber of image frames searched for this change is preferably confinedto those captured close to the estimated time-to-impact (see FIG. 3) inorder to reduce the risk of error.

There is shown in FIG. 7 a flow diagram of a process for identifying theimpact site of a fired round. Using the process described above, awindow is defined around an estimated time to impact. Frames arecaptured from the image sensor during this window. Consecutivelyreceived image frames are subtracted from one another such that eachtime a new frame is obtained a new difference frame is also derived. Thedifference frames are indicative of changes occurring between theassociated consecutive frames given that the subtraction operationremoves stationary background. The difference frames are examined toidentify patches of intensity exceeding a predetermined thresholdintensity. The first difference frame which exhibits a change in excessof a predetermined threshold level is used to determine the location ofthe impact event. More particularly, the impact location is determinedby applying a centroid calculation process to the region of change.

The above process may be modified by computing for each captured imageframe a difference frame by comparing each image frame against areference frame obtained for example prior to firing of the gun.

It will be appreciated that the impact site detection process may beused in combination with the trajectory tracking process and the videoplayback facility described earlier.

A possible architecture for such a combined system is illustrated inFIG. 8. The thermal imaging sensor or camera 13 relays captured imageframes to the gunner's display 14 and to an impact image sequence buffer15. Selected frames are stored in the buffer 15 and can be played backon the display 14. Image frames are also relayed to a damage assessmentprocessor 16 which determines the impact site of a fired round, a rounddetect and track processor 17 which determines the actual trajectory ofa fired round, and to a target detect and track processor 18 which isused to determine motion of a selected target.

A gunner selects a target on his display 14. A ballistic computer 19then predicts the trajectory of a round in order to hit this target,using data obtained by a range estimation system 20 and data from aterrain database 21, and the gun barrel alignment necessary to achievethis trajectory. Under the control of a fire control computer 22, around is then fired. The fire control computer 22 estimates thetime-to-impact for the fired round, and causes the buffer 15 to storeframes in a window surrounding the time-to-impact. The fire controlcomputer 22 also triggers the damage assessment and round detect andtrack processors 16, 17 to look For impact and to track the fired round.This information is subsequently passed to an aimpoint refinementprocessor 23 which recalculates the gun barrel orientation necessary tohit a missed target and updates the ballistic computer. Thisrecalculation takes into account motion of the target determined by thetarget detect and track processor 18.

What is claimed is:
 1. A method of correcting the alignment of a gunfollowing the firing of a round at a target by the gun, the methodcomprising the steps of: aiming the gull at the target and predicting anexpected trajectory for a round to be fired; firing the gun andmonitoring the target and its surrounding area with an image sensor;predicting a plurality of alternative round trajectories which encompasspossible variations from said expected trajectory; analysing image datagenerated by the image sensor to determine which of said alternativetrajectories the fired round followed and, if it is determined that thefired round followed one of said alternative trajectories, determining agun alignment factor from a comparison of said expected trajectory andsaid one of the alternative trajectories followed by the fired round. 2.A method as claimed in claim 1, wherein the image data generated by theimage sensor provides a sequence of image frames which together form avideo record of the travel of the fired round and said step of analysingthe image data comprises normalising the frames to subtract stationerybackground therefrom and then for each said trajectory: mapping thetrajectory onto the two-dimensional plane of the image frames; for eachframe predicting the displacement of a round following the trajectory,relative to a fixed reference point; translating the frames of thesequence relative to said fixed reference point by the respectivepredicted displacements; summing the translated frames to generate asingle cumulative frame; identifying features present in the cumulativeframe which exceed a threshold level and which have a form chosen to beindicative of a fired round.
 3. A method as claimed in claim 2, whereinfor the cumulative frame corresponding to the actual round trajectory,the fired round appears as a bright spot, having a gaussian intensitydistribution and if for one of the trajectories a feature is identifiedin the cumulative image which exceeds said predetermined threshold thenthat trajectory is identified as the trajectory followed by the round,if features are so identified for a number of different trajectories,then the feature having the strongest intensity is selected and theassociated trajectory identified.
 4. A method as claimed in claim 1,wherein the image data provides a sequence of image frames whichtogether form a video record of the travel of the fired round andwherein the step of analysing the image data comprises the steps of:identifying a portion or patch of each frame predicted to contain theround, said patch having the same extent for each frame; normalizing thepatches to subtract stationary background therefrom: and then for eachof the trajectories: mapping the trajectory onto the two-dimensionalplane of each of the patches; for each patch predicting a displacementof a round following the trajectory relative to a fixed reference point;translating the patches of the sequence relative to said fixed referencepoint by the respective predicted displacements; summing the translatedpatches to generate a single cumulative patch; and identifying featurespresent in the cumulative patch which exceed a threshold level and whichhave a form indicative of a fired round, whereby it is only necessary totranslate and sum the identified patches, considerably reducing thecomplexity of the image processing operation.
 5. A method of determiningthe site of impact of a round fired by a gun at a target, the methodcomprising: aiming the gun at the target and predicting an expectedtrajectory for a round to be fired, predicting a plurality ofalternative round trajectories which encompass possible variations fromthe expected trajectory to identify an area surrounding the target wherethe round is likely to impact, monitoring the target and its surroundingarea with an image sensor having an output that is recorded; defining athreshold level of change in the output of the image sensor as beingindicative of an impact of a round; following the firing of the round,detecting a change in the output of the image sensor in excess of saiddefined threshold and identifying the region of change; and determiningthe centroid of said region of change and identifying this centroid asthe site of impact of the fired round.
 6. A method as claimed in claim5, wherein the detected change in the output of the image sensor isdetermined relative to a preceding image frame in a sequence of imageframes.
 7. A method as claimed in claim 5, wherein the detected changein the output of the image sensor is determined relative to an imagerecorded prior to firing of the round.
 8. A target hit assessment methodfor enabling a gun crew to determine the accuracy of a round fired by agun, the method comprising: aiming the gun at the target and predictingan expected trajectory for a round to be fired, predicting a pluralityof alterative round trajectories which encompass possible variationsFrom the expected trajectory, estimating prior to firing the possiblevariations in time-to-impact of the round, from properties of the roundand the gun, prevailing atmospheric conditions and the predictedtrajectories; and following firing of the gun, commencing recording of avideo sequence of the target shortly before the estimatedtimes-to-impact of the round and subsequently stopping recording shortlyafter the estimated times-to-impact; and playing back the recordedsequence at any appropriate rate on a video display to allow theaccuracy of the firing to be quantified.
 9. A method as claimed in claim8, wherein the video sequence comprises less than 50 frames and thesequence is played back, slowed down by a factor of 20 or more.
 10. Atarget hit assessment system for enabling a gun crew to determine theaccuracy of a round fired by a gun, the system comprising: an imagesensor having a field-of-view capable of including an intended target;computer means for predicting an expected trajectory for a round to befired; for predicting a plurality of alternative round trajectorieswhich encompass possible variations from the expected trajectories; andfor estimating the possible variations in the time-to-impact of theround to be fired by the gun with reference to the time of firing of thegun; video recording means coupled to the image sensor and arranged torecord a video sequence from the image sensor commencing shortly beforethe estimated times-to-impact of a fired round and stopping shortlyafter the estimated times-to-impact; and video display means coupled tothe video recording means for receiving therefrom said recorded videosequence for playback.
 11. A method of correcting the alignment of a gunfollowing the firing of a round at a target by the gun, the methodcomprising the steps of: aiming the gun at the target and predicting anexpected trajectory for a round to be fired; firing the gun andmonitoring the target and its surrounding area with an image sensor thatgenerates image data in the form of a sequence of image frames whichtogether form a video record of the travel of the fired round;predicting a plurality of alternative round trajectories which encompasspossible variations from said expected trajectory; analyzing image datagenerated by the image sensor to determine which of said alternativetrajectories the fired round followed and, if it is determined that thefired round followed one of said alternative trajectories, determining agun alignment factor from a comparison of said expected trajectory andsaid one of the alternative trajectories followed by the fired round,wherein the image data is analyzed by normalizing the frames to subtractstationary background therefrom and then for each said trajectory:mapping the trajectory onto the two-dimensional plane of the imageframes; for each frame predicting the displacement of a round followingthe trajectory, relative to a fixed reference point; translating theframes of the sequence relative to said fixed reference point by therespective predicted displacements; summing the translated frames togenerate a single cumulative frame; and identifying features present inthe cumulative frame which exceed a threshold level and which have aform indicative of a fired round.